I meant that I have read that Pb_{3}O_{4} is a mixture of PbO and PbO_{2} So what is the ratio of these in the mixture.

Pb_{3}O_{4} Can carry out the reactions of PbO and PbO_{2} so i need to find the ratio of +2 and +4 in this.

OK, so you can set up two equations x + y = 3 (for atoms of lead) and x + 2y = 4 (for atoms of oxygen), where x is the amount of lead (IV) oxide and y the amount of lead (II) oxide...

Of course, in this case it's relatively easy by trial and error....